DAISY DTB File Issues and Conventions
Validity VS Functionality
Files in a DAISY DTB which is fully valid may be problematic when a CD containing the DTB is read with a player which is built on a consumer device platform, if those files exceed 64 characters in length. This total number of characters includes the file extension. For example, ".mp3" adds 4 characters to the total number of characters in the file name. During the "distribution build" process some DAISY production tools support the use of file names exceeding this limit. If one or more of the mp3 file names exceeds the limit, consumer based players may not play those files.
If the DAISY production tool being used permits file name length greater than 64 characters, procedures should be put in place to ensure that during post production, the 64 character limit is always observed.
Potential Consumer Device Platform Player Response
When a file with a file name exceeding 64 characters is reached, the player may announce that the audio file is not supported. If some files are longer and others are not, it should be possible to "navigate" to the next file that has a file name not exceeding 64 characters. This however is not an acceptable user reading experience.
When a long file name is encounter by a consumer based player, it may "truncate" the file name and thus not 'read' or recognize the actual extension for the file.
Distribution Media Formats
The DAISY 2.02 Specification states:
"3.4 Distribution/storage media formats
Use of a file system that allows long file names (more than 8+3 characters) is required.
As for CD-ROM, use of the 'Joliet' extension to ISO 9660 is recommended. The 'Romeo' file system shall not be used."
Note that there is also a "relaxed" Joliet specification that permits file names longer than 64 characters.
CD Writing (Burning) Software
One problem is that now some CD writing software will allow you to bypass these restrictions, often with little or no warning. The ISO standard file name lengths permitted are:
- ISO 9660 Level 1: 8.3 filenames only
- ISO 9660 Level 2: Up to 31 characters
- ISO 9660 with Joliet extension: Up to 64 unicode characters
In one CD writing software system, for example, the default values for ISO are set to Joliet Relaxed ISO restrictions: Allow path depth of more than 8 directories, Allow more than 255 characters in path. There is a warning at the bottom of the screen stating: "This CD might not be readable under DOS/Windows 3.1x. For full compatibility choose ISO Level 1, ISO 9660 character set, and no relaxed ISO restrictions." DAISY DTBs on CD created with file names of more than 64 characters are not ISO 9660 Joliet compliant CD.
Note that if an organization is producing DAISY DTBs with the goal that they will be playable on all mp3 players, the file name length should be restricted to 8 characters plus the extension (".mp3"). Many mp3 players will play mp3 files with file names up to 64 characters, however, some are limited as noted here.
NCC Heading Length
The 64 character limit on mp3 file names does not limit the number of characters that may be used in the structural headings of the DTB. The file names are truncating during the post production process.
ASCII characters, that is the alpha characters Aa to Zz, digits 0 to 9, underscore "_" and dash "-" are permitted. No other characters, symbols or accents should be used in file names. See The DAISY 2.02 Specification "2.5.3 Audio filenames"
Some of the DAISY players which are built on a consumer device platform are limited in the number of files they can process. Books which contain a very large number of files may result in slow response time, or, in extreme cases, it may not be possible for these players to play the DTB at all. The file number limit varies, but a highly structured DAISY 2.02 DTB with more than 1,300 files and with file names of more than eight characters in length may be problematic.
The number of files in most DAISY DTBs do not approach this limitation. Organizations should however be aware of this.
Text is available under the terms of the DAISY Consortium Intellectual Property Policy, Licensing, and Working Group Process.